The obvious model for those hoping to reverse the fattening of America is the campaign against smoking. Before the surgeon general officially condemned smoking in 1964, rising cigarette consumption seemed an unstoppable trend; since then, consumption per capita has fallen more than 50 percent.

But it may be hard to match that success when it comes to obesity. I'm not talking about the inherent difficulty of the task - getting people to consume fewer calories and/or exercise more may be harder than getting people to stop smoking, but we won't know until we try. I'm talking, instead, about how the political winds have shifted.

Public health activists were successful in taking on smoking in part because at the time corporations didn't know how to play the public opinion game. By today's standards, the political ineptitude of Big Tobacco was awe-inspiring. In a famous 1971 interview on "Face the Nation," the chairman of the board of Philip Morris, confronted with evidence that smoking by mothers leads to low birth weight, replied, "Some women would prefer having smaller babies."

Today's food industry would never make that kind of mistake. In public, the industry's companies proclaim themselves good guys, committed to healthier eating. Meanwhile, they outsource the campaigns against medical researchers and the dissemination of crude anti-anti-obesity propaganda to industry-financed advocacy groups like the Center for Consumer Freedom.

More broadly, the ideological landscape has changed drastically since the 1960's. (That change in the landscape also has a lot to do with corporate financing of advocacy groups, but that's a tale for another article.) In today's America, proposals to do something about rising obesity rates must contend with a public predisposed to believe that the market is always right and that the government always screws things up.

You can see these predispositions at work in an article printed last month in Amber Waves, a magazine published by the Department of Agriculture. The article is titled "Obesity Policy and the Law of Unintended Consequences," suggesting that government efforts to combat obesity are likely to be counterproductive. But the authors don't actually provide any examples of how that might happen.

And the authors suggest, without quite asserting it, that because people freely choose obesity in a free market, it must be a good thing.

"Americans' rapid weight gain may have nothing to do with market failure," the article says. "It may be a rational response to changing technology and prices. ... If consumers willingly trade off increased adiposity for working indoors and spending less time in the kitchen as well as for manageable weight-related health problems, then markets are not failing."

How can medical experts who see obesity as a critical problem deal with an ideological landscape tilted in the direction of doing nothing?

One answer is to focus on the financial costs of obesity, and the fact that many of these costs fall on taxpayers and on the general insurance-buying public, rather than on the obese individuals themselves. (To their credit, the authors of the Amber Waves article do mention this issue, although they play it down.)

It is more important, however, to emphasize that there are situations in which "free to choose" is all wrong - and that this is one of them.

For one thing, the most rapid rise in obesity isn't taking place among adults, who, we hope, can understand the consequences of their decisions. It's taking place among children and adolescents.

And even if children weren't a big part of the problem, only a blind ideologue or an economist could argue with a straight face that Americans were rationally deciding to become obese. In fact, even many economists know better: the most widely cited recent economic analysis of obesity, a 2003 paper by David Cutler, Edward Glaeser and Jesse Shapiro of Harvard University, declares that "at least some food consumption is almost certainly not rational." It goes on to present evidence that even adults have clear problems with self-control.

Above all, we need to put aside our anti-government prejudices and realize that the history of government interventions on behalf of public health, from the construction of sewer systems to the campaign against smoking, is one of consistent, life-enhancing success. Obesity is America's fastest-growing health problem; let's do something about it.